The recent introduction of acetylene-terminated polyimides to produce cured reaction products which are stable at very high temperatures of 450.degree. C. and up has created an interest and need to produce the polyimides at attractive and competitive costs. The prime difficulty in the preparation of the acetylene-terminated polyimides which are described, for example, in U.S. Pat. No. 3,845,018 and U.S. Pat. No. 3,879,349, both to Norman Bilow et al, is the preparation of the monomers which include in one instance the preparation of meta-aminophenylacetylene (APA).
In Ser. No. 840,553 filed in the U.S. Patent Office in the names of E. T. Sabourin and C. M. Selwitz on Oct. 11, 1977, now U.S. Pat. No. 4,128,588, an improved procedure is described for the preparation of precursors to nitrophenylacetylenes (NPA), which NPA is a precursor to APA. In accordance with the teachings of Ser. No. 840,553, nitrophenyl hydroxy substituted acetylenes are prepared from nitrobromobenzene by the reaction of the nitrobromobenzene with a substituted terminal acetylene compound containing at least 3 carbon atoms and a hydroxy group on the carbon atom adjacent to the acetylene group at mild conditions to produce a substantial yield of a nitrophenyl hydroxy substituted acetylene. The reaction occurs in the presence of an amine-type solvent, which serves not only as a solvent but as a complexing agent with the by-product HBr, which is produced during the reaction. The substitution reaction is catalyzed by a complex palladium salt containing two halogen moieties and two substituted phosphine moieties where the substituents on the phosphorus are phenyl, lower alkyl groups and substituted phenyl groups. The catalytic activity of the palladium complex salt is promoted with a small amount of cuprous iodide.
While the process described in Ser. No. 840,553 represents a substantial advance over the prior art techniques which are described in the opening pages of Ser. No. 840,553, the process, on scale-up, was found to be sluggish after initial high reaction rates which eventually resulted in insufficient conversion in normal and desirable commercial reaction times, i.e. less than four hours. In addition, unwanted higher boiling by-products in undesirable amounts were formed, resulting in lowered selectivities of the process to the desired nitrophenyl substituted acetylenes. The unwanted by-products and unreacted charge materials are difficult to remove from the reaction product to produce products of sufficient purity for their direct use in the production of the acetylene terminated polyimides described in the Bilow et al patents referred to above. Product recovery is difficult because the reaction product which contains an NO.sub.2 group is thermally unstable, which prohibits the use of distillation for separation even at reduced pressures or with steam. Fractional crystallization also proved to be unfeasible on products which contained about 15 percent of unreacted charge materials and higher boiling by-products.
It has now been found in accordance with the invention that improved yields of nitrophenyl hydroxy substituted acetylenes can be prepared from nitrobromobenzene by reaction of the nitrobromobenzene with a substituted terminal acetylene compound containing at least three carbon atoms and a hydroxy group on the carbon atom adjacent to the acetylene group at mild conditions. The reaction occurs in the presence of an amine type solvent; a catalyst comprising a complex palladium salt containing two halogen moieties and two substituted phosphine moieties; a promoter comprising a small amount of cuprous iodide; and a sufficient amount of an excess triphenyl phosphine over that amount needed to form the palladium catalyst to result in a yield of desired nitrophenyl hydroxy substituted acetylene of at least 90 weight percent and usually in excess of 95 weight percent. In one preferred embodiment of this invention, the desired nitrophenyl hydroxy substituted acetylene can be recovered in substantially pure form by fractional crystallization of the reaction product.
Any nitrobromobenzene can suitably be employed in the process of this invention. The source of the nitrobromobenzene or its method of preparation are well known in the art and are not critical to the operation of the process of this invention. The suitable nitrobromobenzenes are, of course, the ortho-, meta- and para-nitrobromobenzenes; and of these, meta-nitrobromobenzene is preferred.
The nitrobromobenzene is reacted with a substituted terminal acetylene compound containing at least 3 carbon atoms and a hydroxy group on the carbon atom adjacent to the acetylene group. The preferred substituted terminal acetylene compounds are those having the formula: ##STR1## where R.sub.1 and R.sub.2 can be the same or different and are selected from the group consisting of hydrogen, lower alkyl groups having from 1 to 4 carbon atoms, phenyl, substituted phenyl; or where R.sub.1 and R.sub.2 can form a saturated 5- or 6-membered ring. The preparation of these compounds is well known in the art and forms no part of the subject invention. For example, acetylene can be reacted with acetone to form 2-methyl-3-butyn-2-ol, which is the preferred substituted terminated acetylenic charge stock for use in the process of this invention. Other suitable acetylenic compounds include the following:
3-methyl-1-pentyn-3-ol; PA0 3-ethyl-1-pentyn-3-ol; PA0 2-phenyl-3-butyn-2-ol; PA0 1-ethynylcyclohexanol; and PA0 1-ethynolcyclopentanol. PA0 bis(triphenylphosphine)palladium dibromide; PA0 bis(tri-n-butylphosphine)palladium dichloride; PA0 bis(tri-t-butyl-phosphine)palladium dichloride; PA0 bis(tri-i-butylphosphine)palladium dichloride; PA0 bis(triethylphosphine)palladium dichloride; PA0 bis(tripropylphosphine)palladium dichloride; PA0 bis(tritolylphosphine)palladium dichloride; PA0 bis(trianisylphosphine)palladium dichloride; PA0 bis(tri(chlorophenyl)phosphine)palladium dichloride; and PA0 bis(tri(bromophenyl)phosphine)palladium dichloride.
Usually the nitrobromobenzene is reacted with the terminal acetylene compounds in a molar ratio of about 1:1, but suitable molar ratios include those from 1:0.5 to 1:100 and are more preferably from 1:1 to 1:5.
The reaction of the nitrobromobenzene with the terminal acetylenic compounds defined above occurs in the presence of a dialkyl or trialkyl amine solvent and a complex catalyst system. The amine solvent can suitably have the formula: ##STR2## where R.sub.3, R.sub.4 and R.sub.5 can be the same or different and are selected from the group consisting of hydrogen and lower alkyl groups having from 1 to 4 carbon atoms, with the proviso that no more than one of said R groups can be hydrogen. Suitable solvents include but are not limited to dimethylamine, trimethylamine, diethylamine, triethylamine, ethylpropylamine, ethylbutylamine and dibutylamine.
The catalyst employed is a complex palladium salt containing two halogen moieties, where the halogen is selected from the group consisting of bromine, iodine and chlorine, and two trisubstituted phosphine moieties where the constituents are selected from phenyl, alkyl groups having from 1 to 4 carbon atoms, and substituted phenyl groups. A suitable palladium complex would have the formula: ##STR3## where x is bromine, iodine or chlorine, and R, R' and R" are the same or different and are selected from the group consisting of phenyl, alkyl groups having from 1 to 4 carbon atoms and substituted phenyl groups. The substituents on the phenyl groups can include alkyl groups having from 1 to 4 carbon atoms, alkoxy groups having 1 to 4 carbon atoms, and halogen. A suitable list of representative palladium complex salts which can be employed in the process of this invention include:
The palladium catalyst can be added to the reaction mixture as such or can be formed in situ in the reaction mixture by the separate addition of a palladium salt having the formula Pd(X).sub.2 where X is as defined, and a trisubstituted phosphine compound having the formula: ##STR4## where R, R' and R" are as defined and wherein the molar ratio of the trisubstituted phosphine to the palladium is about 2:1. It has been found in accordance with the invention that the presence of excess trisubstituted phosphine, e.g. triphenylphosphine, over and above that necessary to form the palladium catalyst results in an overall yield of the desired nitrophenyl substituted acetylenes of over 90 weight percent, usually over 95 weight percent. Another manner of stating the above is that the addition of the excess trisubstituted phosphine results in reduced total reaction times for substantially complete reaction of the nitrobromobenzene and/or an improved selectivity to the formation of the desired nitrophenyl substituted acetylenes.
Whether the palladium catalyst is formed in situ or whether the palladium catalyst is formed separately and added to the reaction system, the molar ratio of the trisubstituted phosphine compound to palladium in the reaction system must be above 2:1, and is usually from 2.5:1 to 50:1, more preferably from 2.5:1 to 25:1, and most preferably from 5:1 to 20:1. It has been found that as the molar ratio of trisubstituted phosphine to palladium increases over about 20:1, both the overall reaction time tends to increase and the selectivity tends to decrease.
A promoter for the catalyst system is also employed, and this promoter comprises cuprous iodide. Usually the amount of the promoter is very small, and suitable amounts of promoter include a molar ratio of promoter to palladium catalyst of from 0.5:1 to 20:1, preferably from 1:1 to 5:1. The amount of the palladium catalyst employed in the reaction is usually from 0.01 to 1.0 mole percent based on nitrobromobenzene and is more preferably from 0.02 to 0.05 mole percent based on nitrobromobenzene.
The reaction of the nitrobromobenzene with the acetylene-terminated compound is really a substitution-type reaction, and the reaction conditions to employ are relatively mild and include a temperature from about 20.degree. to 200.degree. C. and more preferably from 50.degree. to 125.degree. C. However, it is considered that the reaction conditions are not critical, and the precise reaction conditions to employ would be obvious to one having ordinary skill in the art. The reaction conditions should be such that the solvent chosen is maintained in the liquid phase. The normal reaction pressure is atmospheric; however, increased reaction pressures of up to 250 psig (1.7 MPa) or higher can be employed. The reaction time to employ is somewhat dependent on the particular charge stock and catalyst chosen and, of course, on the reaction temperature. Usually the reaction time is from 1 hour to 20 hours, but is more usually from 1 hour to 12 hours. Higher or lower reaction times can be employed, for timing is not a critical parameter but rather in many cases serves to increase the yield of the desired reaction product.
A typical reaction sequence is shown in Equation 1 below, which utilizes certain specific charge stocks which fall within the scope of the charge stocks defined above. ##STR5##
Referring to Equation 1 above, it can be seen that a by-product of the reaction sequence is HBr. The HBr cannot be permitted to remain in the reaction product because of its corrosive nature.
It is one of the purposes of the amine solvent to react with the HBr in order to produce the amine hydrobromide salt and render it inactive. The amount of the amine solvent to employ in the reaction is not critical but must thus be sufficient to maintain the reactants in the liquid phase plus provide sufficient amine to react with the by-product HBr. Amounts of solvent from 500 to 700 ml per mole of nitrobromobenzene have successfully been employed. However, greater or lesser amounts can be employed, and the particular amount to employ would be within the normal skill in the art given the criteria set forth above.